Several optical, non-contact gauges presently exist. These gauges typically measure the outside diameter of wire, tubing, fibers, cylinders, small plates and other objects whose outside dimension is of interest. The non-contact feature is important where the object can easily be deformed using ordinary contact caliper devices. High accuracy, repeatability and resolution are also prime features of these non-contact optical gauges. These devices often employ a shadow cast technique whereby a collimated white light of parallel rays or other radiation source is interrupted by the object to be measured. The interrupted light or shadow, along with its surrounding light, are cast upon a light sensing element which discriminates between them. The discrimination technique usually measures total amplitude differences as a function of the presence or absence of radiation in time.
Shadow cast and shadowgraphy have been, in the past, used in the machine tool industry to visually inspect parts such as screw threads. By placing the part in front of a light projector, the part's shadow would be enlarged on a wall or screen for easier inspection by the eye. In today's non-contact optic gauges, the same principle applies, except that the projected light is collimated and an electronic sensor acts as the screen.
The radiation sources which are currently available for such gauges are generally of two types: the constant incandescent sources and highly focused laser beams. In the incandescent version, a light source can be collimated by shining it through a pin hole and then through an objective lens. A parallel light emerges which provides a shadow when disposed on the object to be measured. In some gauges, the relative intensity of the light as a function of the shadow provides an analog means of determining the outside dimension. In other gauges, mechanical shutters are moved to scan the dark and light area in time. In others the on/off state of a photo diode array is used.
With respect to laser beam devices, gauges have been known to include a laser beam for striking a rotating mirror which, in-turn, fans out the beam through an objective lens. See Petrohilos, U.S. Pat. No. 3,905,705, which is hereby incorporated by reference. This results in a fast moving parallel beam across an object to be measured. The receiving sensor of such devices, as in the case of the incandescent versions, measures relative radiation strength or a time varying proportional signal.
In such prior art methods described above, the mechanical alignment of the optics, mirrors, shutters, sensors and/or radiation sources is critical. Consequently, precision manufacturing fixtures and techniques are generally required. High quality optical material and sophisticated sensors and circuitry are employed to prevent non-linearities in the measurement. Even with these precautions, inaccuracies, and non-linearities still exist to a certain degree. Several patents address further compensation means to reduce these remaining inaccuracies. But these usually require additional or secondary hardware and sensors to compensate for the inherent inaccuracies. Finally, heat generated by the incandescent or laser sources and the measurement and compensation circuitry requires extensive heat sinking and isolation. Often, an abundant power source, which in turn generates considerable heat, is required to feed the radiation source and electronics. The resultant high heat is wasteful and affects instrument accuracy over time.
Accordingly, there is a need for a less expensive and more efficient non-contact optical gauge which compensates for inherent inaccuracies and non-linearities of the optical system. Secondly, there is also a need for a low heat design for such instruments that is compact in size and requires little or no maintenance.